The invention relates to an energy storage device, a system comprising an energy storage device and a method for generating a supply voltage of an energy storage device, in particular in a battery direct converter circuit when starting or starting up an electrically operated vehicle.
The trend is that in the future electronic systems which combine new energy storage technologies with electrical drive technology will be used increasingly both in stationary applications, such as e.g. wind power installations or solar installations, and in vehicles, such as hybrid or electric vehicles.
FIG. 1, for example, shows the feed of alternating current into a three-phase electric machine 101. In this case, a DC voltage provided by a DC voltage intermediate circuit 103 is converted into a three-phase AC voltage by means of a converter in the form of a pulse-controlled inverter 102. The DC voltage intermediate circuit 103 is fed by a line 104 of battery modules 105 connected in series. In order to be able to meet the requirements for power and energy provided for a respective application, often a plurality of battery modules 105 are connected in series in a traction battery 104. An energy storage system of this type is often used for example in electrically operated vehicles.
The series circuit comprising a plurality of battery modules is associated with the problem that the entire line fails if a single battery module fails. Such a failure of the energy supply line can result in failure of the entire system. Furthermore, temporarily or permanently occurring power reductions of an individual battery module can result in power reductions in the entire energy supply line.
The document U.S. Pat. No. 5,642,275 A1 describes a battery system with an integrated inverter function. Systems of this type are known under the name Multilevel Cascaded Inverter or else Battery Direct Inverter (BDI). Such systems comprise DC sources in a plurality of energy storage module lines, which can be connected directly to an electric machine or an electrical power supply system. In this case, single-phase or polyphase supply voltages can be generated. The energy storage module lines in this case have a plurality of energy storage modules which are connected in series, wherein each energy storage module has at least one battery cell and an assigned controllable coupling unit, which makes it possible to interrupt the respective energy storage module line or to bridge the respectively assigned at least one battery cell or to switch the respectively assigned at least one battery cell into the respective energy storage module line, depending on control signals. By suitable driving of the coupling units, for example with the aid of pulse width modulation, suitable phase signals for controlling the phase output voltage can also be provided, with the result that a separate pulse-controlled inverter can be dispensed with. The pulse-controlled inverter required for controlling the phase output voltage is thus integrated into the BDI, as it were.
BDIs usually have a higher efficiency and a higher degree of failsafety in comparison with conventional systems, as shown in FIG. 1. The failsafety is ensured, inter alia, by virtue of the fact that defective, failed or not fully effective battery cells can be disconnected from the energy supply lines by suitable bridging driving of the coupling units.
In both systems, both in the system shown in FIG. 1 and in a BDI, it can happen, precisely in the case of use in electrically operated vehicles, that the battery cells are to be operated at low ambient temperatures, for example in winter. Customary batteries, such as lithium-ion batteries, for example, have a temperature-dependent internal resistance. At low temperatures, the internal resistance is increased, and so under certain circumstances the full power cannot be drawn from the battery. At low ambient temperatures, therefore, depending on the composition of the battery cells used, the battery cells have to be preconditioned, that is to say that the battery cells have to be heated, in order that the power required for driving, in particular for starting up, can be provided. The heating process is energy- and time-intensive in this case.